Corrosion of Ceramic and Composite Materials Part 11 pot

Corrosion studies of various metal fluoride glasses in liquid water: application to fiuoride-... To obtain these optimum characteristics, it issometimes required to coat the reinforcemen

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6.64 Walters, H.V Corrosion of a borosilicate glass by

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6.65 Metcalfe, A.G.; Schmitz, G.K Mechanism of stress corrosion

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6.66 Priest, D.K.; Levy, A.S Effect of water content on corrosion

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6.67 Koch, G.H.; Syrett, B.C Progress in EPRI research on

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6.68 Velez, M.H.; Tuller, H.L.; Uhlmann, D.R Chemical durability

of lithium borate glasses J Non-Cryst Solids 1982, 49 (1–

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6.69 Conzone, C.D.; Brown, R.F.; Day, D.E.; Ehrhardt, G.J In vitro and in vivo dissolution behavior of a dysprosium lithium borate glass designed for the radiation synovectomy treatment

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6.70 Day, D.E Reactions of bioactive borate glasses with

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6.71 Yoon, S.C Lead release from glasses in contact with beverages; M.S thesis, Rutgers University, New Brunswick,

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6.72 Pohlman, H.J Corrosion of lead-containing glazes by water

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6.74 Lehman, R.L.; Yoon, S.C.; McLaren, M.G.; Smyth, H.T Mechanism of modifier release from lead-containing glasses

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6.88 Doremus, R.H.; Bansal, N.P.; Bradner, T.; Murphy, D Zirconium fluoride glass: surface crystals formed by reaction

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is manufactured in an attempt to obtain the best properties oftwo materials or at least to capture a specific property of eachmaterial that is potentially better in the composite It is alsopossible for the composite to have a particular property thatneither component exhibited individually According to Holmesand Just [7.1], a true composite is where distinct materials arecombined in a nonrandom manner to produce overall structuralcharacteristics superior to those of the individual components.Although, in a very broad sense, products such as glazed

ceramic tile, enameled metal, and ceramic coated metal (e.g.,thermal barrier coatings) could be considered composites, theywill not be considered as such here Only those materials where

a substantial intermixing of the different materials exists on amicroscopic scale will be considered composites

The concept of composite materials is not a new idea and isdefinitely not limited to ceramics Nature has provided us withseveral excellent examples of composite materials Wood is acomposite of cellulose fibers contained in a matrix of lignin.Bone, another example, is composed of the protein collagenand the mineral apatite In all these materials, the result is aproduct that is lighter and stronger than either of thecomponents individually Because of this, they can be used inmore severe environments, e.g., space exploration A list ofthe more desirable properties of a composite is given in Table7.1 In a very broad sense, all engineering materials arecomposites of one kind or another

The matrix and the reinforcement, quite often fibrous,provide two different functions The reinforcement is mostoften a discontinuous phase whether it be a fibrous material

or a particulate material It is important that the reinforcement

be discontinuous, especially if it is a ceramic, so that cracks

TABLE 7.1 Desirable Properties of Composites

will not be able to propagate through it The matrix must notdamage the reinforcement and it must transmit any stresses tothe reinforcement Thus the adhesion of the matrix to thereinforcement is of prime importance for mechanical integrityand is the region of greatest importance related to corrosion.Since it is necessary to have weak interfaces to maximizetoughness (i.e., resistance to crack propagation), thedevelopment of optimum fiber/matrix interfaces is quitedifficult To obtain these optimum characteristics, it issometimes required to coat the reinforcement fibers withvarious materials to obtain the proper debonding, sliding, and/

or reaction characteristics Fibers that do not debond do notenhance toughening and lead only to increased brittle fracture

of the composite [7.2–7.7]

A recent development in composites is that of a nanosizedsecond phase or reinforcement material The second phaseparticles are generally less than 300 nm and are present inamounts equal to 1–30 vol.% These new composites

unfortunately have been called nanocomposites.

Before going into the specifics of corrosion of compositematerials, a few words must be said about those materials that

have been called cermets Historically, the term cermet was

derived to cover those materials composed of cobalt-bondedtungsten carbide and used as cutting tools Since cermetscontain both ceramics and metals, some confusion has existed

in the literature as to an exact meaning The term, however,has been used to cover a broad list of materials It appears thatthe ceramic community confines cermets to essentially cuttingtool materials, whatever the matrix or reinforcement, whereasthe metals community confines cermets to only those materialswith a metal matrix Since the broader concept of compositesincludes those materials called cermets, only the term compositewill be used in the discussion below

The actual corrosion of composite materials quite oftenbegins with reaction of the reinforcement material and

especially with any interface material (called the interphase)

used to coat the reinforcement for debonding One property

that exacerbates this is a mismatch in thermal expansioncoefficients between the reinforcement and the matrix, leading

to microcracks These microcracks allow the ingress ofcorrosive gases (e.g., oxygen) Courtright [7.8] has given thevalue of 10-12 g O2/ cm sec for the limit of oxygen ingress thatcauses nonoxide fiber deterioration Microcracks are also quiteoften a product of sample preparation techniques, and thusgreat care must be used in cutting and grinding/polishingsamples for testing If the composite is cut or machined, anyexposed fiber reinforcement will be susceptible to attack bythe environment Because of this inherent problem, protectivecoatings are often applied to the exterior surfaces Actually,the whole corrosion process of composite materials is not unlikethat of other polyphase ceramic materials where the grainboundary phase is the first to corrode A completeunderstanding of all the phases that make up the microstructure

of the composite must also be known for an accurateinterpretation of any corrosion For example, Munson andJenkins [7.9] reported that their samples were actually attackedinternally by molten metal from a small amount of freealuminum present as a residue during the manufacture ofDimox™* (a melt-infiltrated alumina) Actually, a largeamount of the literature on composites is concerned with anevaluation of the internal reactions that take place among thevarious reinforcement, interphase, and matrix materials Thetime-dependent loss of strength due to the corrosive nature ofmoist environments at room temperature is a major concernfor composites containing glass or glass-ceramics as either thematrix or the reinforcement [7.10] As temperatures areincreased, the concern shifts toward oxidation problemsassociated with nonoxide materials See the discussions inProperties and Corrosion, for more details of oxidation andits effects upon the properties of nonoxides

* DIMOX™ (directed metal oxidation) is the name given to composites manufactured by a process developed by Lanxide Corp., Newark, DE in 1986.

Chap 5, Sec 5.2.2, Nitrides and Carbides, and Chapter 8,

With the advancement of the development of composites,there is an increasing number of acronyms with which onemust contend To aid the reader, a list is given in Table 7.2 ofthe most common acronyms.

nitrides, and carbides either in the amorphous or crystallinestate The surface chemistry and morphology of fibers is veryimportant in determining their adherence to the matrix Fiberinternal structure and morphology determines the mechanicalstrength A tremendous amount of literature is available thatdiscusses the degradation of mechanical properties astemperatures are increased in various atmospheres; however,there is very little interpretation of any corrosion mechanismsthat may be involved Although many composites are classified

as continuous-fiber-reinforced, some composites contain fibersthat are actually not continuous but of a high aspect ratio (i.e.,length-to-width) The actual matrix material will determinethe aspect ratio required to obtain a certain set of properties.Thus the term “high aspect ratio” is a relative term

Boron fibers can generally be heated in air to temperatures

of about 500°C without major strength deterioration Above500°C, the oxide that formed at lower temperatures becomesfluid increasing the oxidation rate and drastically reducing thestrength [7.11] Galasso [7.11] discussed the benefits of coatingboron fibers with either SiC or by nitriding the surface TheSiC coating was more protective than the nitride with strengthretention even after 1000 hr at 600°C in air Boron carbide(B4C) is stable to 1090°C in an oxidizing atmosphere, whereasboron nitride is stable to only 850°C

Carbon or graphite fibers have been used since the early1970s as reinforcement for composites Strength loss due

to oxidation occurs at temperatures above 500°C in air Aninteresting structural feature of carbon fibers is that theyhave a relatively large negative axial thermal expansioncoefficient

Glass fibers generally are used as reinforcement forcomposites that are to be used at low temperatures (i.e.,

<500°C) due to the softening of glasses at elevatedtemperatures These composites are generally of the polymermatrix type and are used for marine or at least moistenvironments It is well known that glass is attacked by moistenvironments with the specific mechanism dependent upon

Schmitz [7.12] that borosilicate glass fibers when exposed tomoist ambient environments developed surface tensile stressescaused by exchange of alkali for hydrogen sufficient to causefailure.

A large portion of the CMC today contains SiC fiberreinforcement This is mainly due to the excellent properties

of SiC—low reactivity to many matrix materials, its strength

at elevated temperatures, and its oxidation resistance It is thislatter property (i.e., oxidation resistance) that generally causesdeterioration in these materials SiC will oxidize readily whenheated to temperatures greater than 1000°C As discussed inoxygen, active corrosion takes place with the formation ofgaseous products of CO and SiO At higher partial pressures,passive oxidation occurs with the formation of CO and SiO2that may be protective if cracks do not form The formation ofcracks is dependent upon the heat treatment and whether theoxide layer is crystalline or amorphous These reactionsgenerally result in the decrease of fiber strength Nicalon™fiber*, being formed by the pyrolysis of organometallics,actually contains some remnant oxygen (~9%) and carbon(~11%) that will affect the subsequent oxidation of the fiber.Two different grades of Nicalon™ fiber have been examined

by various investigators [7.13–7.15] Clark et al [7.13] reportedthese fibers to exhibit weight losses of 13% and 33% afterbeing treated in argon at 1400°C Both grades of fiber gainedweight (on the order of 2–3%) when treated in flowing wet air

at 1000°C, 1200°C, and 1400°C As-received Nicalon™ fibershave protective sizing (i.e., polyvinyl acetate) on their surfaces.When heated in air, this sizing will burn off at temperaturesbetween 250°C and 500°C At temperatures above about1250°C, the SiCxOy amorphous phase contained in these fibersdecomposed to SiO and CO [7.16]

* Nicalon™, Nippon Carbon Co., Tokyo, Japan.

the pH (see Chap 6) It has been shown by Metcalfe and

Chap 5, Silicon Carbide, page 223, at low partial pressures of

Titanium nitride (TiN) resists attack from iron or nickelaluminides better than does SiC and thus is a betterreinforcement for these metal alloy matrix composites [7.17].

7.2.2 Fiber Coatings or Interphases

Protective coatings (also called interphases) such as graphite

or BN, in addition to providing proper debonding and pullout[7.18,7.19], are used to provide some degree of oxidationresistance [7.20,7.21] for fibers such as SiC Bender et al [7.21]concluded that the BN protects the SiC fiber from the matrixsince BN will not react with SiO2, which is generally present

on the surface of the fibers Boron nitride-coated mullite,carbon, and SiC fibers were tested in a mullite matrix withvarying degrees of success by Singh and Brun [7.22]

Boron nitride-coated SiC fibers have shown a slightimprovement over carbon-coated fibers with an increase of aboutdiscussion concerning embrittlement) temperatures [7.23] Sincesome matrices are grown in situ, techniques to coat fibers becomeproblematic A combination coating of BN and SiC wasdeveloped by Fareed et al [7.24] to eliminate the undesirablereaction of molten aluminum in contact with Nicalon™ fibersforming alumina and aluminum carbide during the directedmetal oxidation method (at 900–1000°C) of forming an aluminamatrix When used alone as a coating, BN oxidation inhibitedcomplete oxidation of the aluminum In combination with SiC,however, Fareed et al believed that any oxidation of BN led tothe formation of boria glass that acted as a sealant to anymicrocracks, thus minimizing oxygen ingress and protection ofthe composite The SiC outer coating protected the BN innercoating during growth of the matrix Ogbuji [7.25] reportedthat the BN first oxidized to B2O3, which then dissolved some

of the SiC fiber and matrix forming a borosilicate liquid If anymoisture were present, the boria may be volatilized by hydrolysisreleasing B(OH)4 gas This reaction resulted in a silica residuethat cemented the fibers together embrittling the composite.100–200°C in composite embrittlement (see Sec 7.3 for a

In an effort to find an interphase or coating for aluminaand mullite fibers, Cooper and Hall [7.26] developed a syntheticfluorophlogopite*, based upon their geochemical approach,that when reacted with alumina formed an intermediate spinelphase that was stable after heating to 1200°C in air for 150 hr.Thus by coating alumina fibers with spinel and then using thefluorophlogopite as an interphase, an alumina matrixcomposite proved successful Above about 1280°C, the aluminareaction with fluorophlogopite produced forsterite, leucite, andspinel along with the volatile fluorides SiF4, AlF3, and KF [see

Eq (7.1) below], making the spinel-coated alumina fiber/fluorophlogopite laminate unstable at those high temperatures.Reactions between mullite and fluorophlogopite formedcordierite in addition to the phases mentioned above This wasnot successful as a mullite fiber composite since the cordieriteallowed potassium diffusion from the fluorophlogopitecontinually deteriorating the mullite In the alumina fiber case,the spinel coating acted as a barrier to potassium diffusion

(7.1)

Cooper and Hall reported that reaction (7.1) occurred attemperatures above 1230°C in flowing dry argon, althoughthermodynamic calculations indicated that the reactionproceeded only after the temperature reached 1279°C Thiswas attributed to the partial pressures of the gaseous phasesnot summing to 1 atm during the experiment in flowing argon

itself through the oxidation of the interface between the fiberreinforcement and the matrix subsequently causing a strongbond between the two leading to embrittlement Embrittlementmay not be noticed when samples are tested in flexure due toload redistribution, thus requiring that samples be tested intension [7.29].

A 21 vol.% SiC in alumina composite was reported byBorom et al [7.30] to form a reaction zone upon oxidation at1530°C for 150 hr that contained mullite and an amorphousaluminosilicate phase containing bubbles from the formation

of CO The SiO2 formed by the oxidation of the SiC reactedwith the alumina matrix to form the mullite It is importantthat the formation of silica in the outer layer is sufficient forcomplete conversion of the alumina to mullite [7.31].Insufficient silica causes a rigid scale that delaminates Toomuch silica forms a scale containing mullite and silica on analumina substrate that may also delaminate due to expansionmismatch during thermal cycling A matrix of mullite worksmuch better than alumina since the scale is more compatiblewith the substrate, both containing mullite, and thus forms aprotective layer Luthra [7.32] reported that the products ofreaction of SiC with alumina should be mullite and aluminawhen the SiC content is below 24.4 vol.% and silica and mullitewhen it is greater than 24.4 vol.% In practice, this limit willvary due to mullite forming over a range of compositions

A TiN/Al2O3 composite was reported by Mukerji and Biswas[7.33] to exhibit linear oxidation kinetics above 820°C after a

short (<120 min) parabolic induction period The change fromparabolic to linear kinetics was reported to be due to thedifference in specific volumes between TiN and TiO2 thatcaused an expansion of the oxidized layer forming cracks,which allowed oxidation to continue The rutile that formedabove 820°C was reported to grow epitaxially with apreferential growth direction of [211] and [101] At 820°Cand 710°C, this oriented growth was not present Tampieriand Bellosi [7.34] reported this oriented growth to occur inthe [221] and [101] directions and only above 900°C Contrary

to Mukerji and Biswas, Tampieri and Bellosi reported parabolicgrowth between 900°C and 1100°C for times up to 1200 min.These differences must be attributed to differences in startingmaterials and experimental conditions since the authors didnot report any specific reasons that one may assign to thevariation in results

TiN decomposes to form titanium oxides and aluminumtitanates at temperatures in excess of 1550°C [7.35] TiN willalso react with alumina to form titanium oxides and aluminumtitanates at temperatures as low as 1450–1500°C Thereforeduring processing by hot pressing, the temperature must bekept below 1500°C and the pressure must be high

The oxidation at 1500°C of TiC-containing (25 vol.%)alumina matrix composite has been reported to form Al2TiO5

as the reaction product by Borom et al [7.31] Approximately

a 30-vol.% expansion accompanied this reaction that causeddelamination of the oxide reaction product layer

In a previous study, Borom et al [7.30] reported theoxidation at 1520°C of MoSi2 (10 vol.%) dispersed within amatrix of alumina to form a reaction layer of mullite andvolatile MoO3 that completely escaped It was suggested thatthis reaction layer contained an interconnected network ofporosity through which the MoO3 escaped, although noevidence of such porosity was given Linear growth kineticswas reported for the formation of this nonprotective layer ofmullite A unique-appearing periodic change in density(porosity) was developed at about 200-µm intervals within